According to audio of the private event obtained by CNN, Heck seemed to recognize that his decision to abandon Trump had become a sore subject among some of his core supporters, calling it a "very difficult situation." About six minutes into the recording he defended his decision to rescind his Trump endorsement by saying that the GOP nominee's "pattern of behavior" was bound to "hand" the White House to Hillary Clinton.
"I want to support him, I really do," Heck, a Nevada Republican, said during a 90-minute discussion during a Las Vegas fundraiser for his Senate campaign.
"But he has got to change his tone and he's got to be -- I don't want to make him into a politician or make him into the same thing he is running against -- but he has got to realize he is not going to win this race by appealing to the 20% or 30% of the Republican base," Heck added.
Heck, who noted that Trump had "one more debate chance" to turn around his candidacy in Las Vegas this week, added, "At the very least, if I have to go with my own conscience to make this right, I am going to vote for Michael Pence as vice president."
Nevada has no write-in option.
The candid assessment gives a glimpse into how Trump has made life difficult for down-ticket Republicans, who have long sought to avoid much public discussion about their controversial nominee. And Heck's private comments are his most detailed views yet in the aftermath of the leaked video of Trump making vulgar and sexually aggressive comments about women during a 2005 taping of a segment on "Access Hollywood," prompting Heck to call on the GOP nominee to withdraw from the race.
Yet Heck has come under attack from his Democratic opponent, Catherine Cortez Masto, for waiting until the video to drop his Trump endorsement, even though the Republican candidate has offended other voting blocs, including Hispanics, for months. At the same time, he has endured sharp backlash in conservative circles, underscoring the challenges Republican candidates face in wooing supporters of the GOP nominee while distancing themselves from his incendiary rhetoric.
During the fundraiser, Heck said Republicans "cut a lot of slack" to Trump because "he is not an accomplished campaigner" and he needed "to learn the ropes" as he ran as a "change agent." And Heck suggested that had Trump been a more disciplined campaigner, he may have been more forgiving to the GOP nominee in the aftermath of the "Access Hollywood" video.
"What has happened is despite who has tried to help him, by focusing his message on where it needs to be to beat Hillary Clinton, he cannot stay on the message," Heck said. "There's only two issues he needs to talk about: the economy and national security. Nothing else matters. And if he would talk about those two issues, he would clean her clock and a lot of people including close personal friends of his have tried to get him to do that."
Heck's 'worst fear'
At the fundraiser, Heck said that his "worst fear" is that the top-of-the-ticket will depress Republican turnout on Election Day. And he believes if Trump loses Nevada by 10-12 points, "it's going to be hard," predicting that he could outperform Trump by 5-6 points.
"We never thought that the House was in jeopardy, that we could lose 30 seats in the House," Heck said. "If the current trajectory continues that becomes a possibility."
Heck spokesman Brian Baluta downplayed his boss' comments at the fundraiser, saying they were consistent with the message he was giving publicly.
"It's unfortunate we've reached the point where it's considered news when someone running for office says the same thing in private that they do in public," Baluta said. "Dr. Heck's position is clear. The same cannot be said for Catherine Cortez Masto, who turns a blind eye to wrongdoing by Hillary Clinton and Harry Reid."
At a debate last Friday, Heck cited his service as an emergency room doctor and as a member of the Army Reserve -- even invoking his wife as a victim of domestic abuse in a prior relationship -- when discussing Trump's "Access Hollywood" comments that many liken to sexual assault.
"The decision I made was an extremely personal decision," Heck said at the debate.
Not just the video
At the fundraiser, Heck called Trump's comments on the video "reprehensible," citing his wife and two daughters and saying his decision to pull the endorsement was "morally and ethically right to me and my family."
But he said that the leaked tape alone didn't force him to withdraw his support of Trump. The congressman suggested that had Trump been focusing "on the issues" while the tape leaked, he may have approached the situation differently.
"Look, it wasn't this video that made the final decision, alright? He's had this pattern of behavior that's persisted... even while he was on the campaign," Heck said. "It's one thing if it happened 11 years ago and now he's concentrating on the issues and talking the way we need to talk to him about it."
In his remarks to donors, Heck said that he's sympathetic to Trump's desire to engage in political brawls -- but said such tactics have undermined his candidacy by taking him off-message.
"I mean, I am a New Yorker, I understand," Heck said. "It's not in his nature. Alright, if he's attacked he wants to punch back. He wants to punch back hard. Look what he's doing is shoring up the Republican base and that's great, but you know cannot win a federal race nationwide with just the Republican base, you've got to have a big tent and he doesn't. And that's why he's floundering in the polls."
Yet, Heck said about 59 minutes into the recording that he did agree with "certain policy stances" of Trump's, while disagreeing with others like trade.
"Ronald Reagan said that my 80% friend is not my 20% enemy," Heck said. "Nobody agrees on 100% on everything."
Heck's decision to pull his endorsement amounted to a sharp reversal; in September the Nevada Senate candidate told CNN he supports Trump.
"Why wouldn't I?" he said when asked if he trusted Trump with the nuclear codes.
Now, Heck suggested, Trump may never occupy the Oval Office.
"He is going to hand the White House to Hillary Clinton," Heck told his donors.